This is a phase I/II randomized, double-blind, placebo-controlled study of safety, tolerance, pharmacokinetics, and anti-HIV activity of PMPA prodrug administered orally to HIV-infected patients with CD4 cell counts > 200 cells/mm3. Fifty patients (10 patients per dose level) at 3 centers will receive PMPA prodrug at a dose of 75, 150, 300, 600, or 900 mg or matching placebo tablets. Sequential cohorts of 10 patients (8 active drug and 2 placebo recipients) will be enrolled at each dose level in a dose-escalating fashion. Patients will be given a dose of PMPA or placebo and observed for one week. Then, patients will be given tablets of PMPA or placebo daily for four weeks. The study will define a maximum tolerated dose.